1. Field of the Invention
This invention relates generally to optical and magneto-optic data storage systems, and in particular to optical and magneto-optic storage systems utilizing a fully or partially transmissive data storage medium.
2. Description of Related Art
Optical and magneto-optic data storage systems store great quantities of data on a disk. The data is accessed by focusing a laser beam onto the disk and detecting the reflected light beam. Three kinds of systems are well known. The first kind is a ROM (Read Only Memory) system in which data is permanently embedded as marks in the disk. The data is detected as a change in reflectivity as the laser beam impinges on the data marks.
The second kind is a WORM (Write-Once-Read-Many) system that allows the user to write data by making marks, such as pits, on a blank optical disk surface. Once the data is recorded onto the disk it cannot be erased. The data in a WORM system is also detected as a change in reflectivity.
The third kind is a WREM (Write-Read-Erase-Many) system in which a laser beam is used to heat a magneto-optic data layer to a critical temperature, generally just below the Curie temperature, in order to write and erase the data. As the critical temperature is reached, the magnetic domain field strength is reduced to close to zero. An external magnetic field, using an electromagnet or a permanent magnet, is then used to reverse the domain to record a one or a zero. In general data is recorded by orienting the magnetic domain of a spot in either an up or a down position. The WREM system reads the recorded data by directing a low power (reading) laser beam to the data layer. The difference in magnetic domain directions causes the plane of polarization of the light beam to be rotated one way or the other, clockwise or counterclockwise. This change in orientation of polarization is then detected and recognized as either a digit "1" or "0".
U.S. Pat. No. 5,255,262 to Best et al., titled "Multiple Data Surface Optical Data Storage System With Transmissive Data Surfaces", issued on Oct. 19, 1993, which is incorporated herein by reference, describes an optical data storage system that includes a multiple data surface medium and an optical head. The medium includes several substrates that are separated by a light transmissive medium. With the exception of the last data layer, data surfaces are substantially light transmissive and are located on the substrate surfaces which lie adjacent to a light transmissive medium. The optical head includes an aberration compensator to allow the head to focus onto the different data surfaces, and a filter to screen out unwanted reflected light.
This patented system is relatively complicated and its manufacture is labor intensive. Also, the system uses a focusing mechanism, which adds complexity and cost, and reduces the system signal to noise ratio.
Therefore, there is still a greatly unsatisfied need for an optical system and a magneto-optic data storage system with a reduced number of components, thus simplifying the overall structure and reducing its cost.